Optical recording media such as compact discs (“CDs”) or digital video discs (“DVDs”) are used in the art to store prerecorded information as a pattern of pits formed in a metallic substrate. The pit patterns are spirally arranged on a disc, and form digital words that can be read by radiating a laser beam on the disc surface and detecting the reflected beam. Each spiral of pits forms a track, which may not physically exist. The optical recording mediums include read-only type discs such as CD-ROM and DVD-ROM, write-once-read-many (“WROM”) type discs such as CD-R (recordable) and DVD-R, DVD+R, and rewritable discs such as CD-RW and DVD RAM, DVD+RW, DVD-RW.
FIG. 1 shows a conventional optical structure 10 in the art for data reproduction. Referring to FIG. 1, structure 10 includes a disc 12, a laser diode 14, a beam splitter 16, a lens 18, and a photo-detector 20. Laser diode 14, beam splitter 16, lens 18, and photo-detector 20 together forms an optical pickup (“OPU”). Disc 12 includes at least one surface 12-2 where pits are formed. Laser diode 14 functions to serve as a laser source for radiating a laser beam 14-2. Beam splitter 16 reflects laser beam 14-2 toward lens 18, and bypasses the reflected beam from lens 18 on photo-detector 20. Lens 18 functions to serve as a scanning device to read data from disc 12, or record data on disc 12. The OPU is generally held by a sled motor (not shown), and is able to move generally perpendicularly with respect to a recording track of disc 12. The OPU may scan disc surface 12-2 along a track in a regulation mode, i.e., in a tangential direction of disc 12, or jump over a plurality of tracks in a search mode, i.e., in a radial direction of disc 12. The quality of data reproduction significantly depends on the position of lens 18. Photo-detector 20 may include photo-detector elements A, B, C and D to detect the positions of a laser spot 22 focused thereon and a pit image 24.
To control the position of lens 18, a servo control system is typically provided to provide servo error control signals that are fed back to the servo control system. The servo error control signals may include a tracking error signal (TE), a focus error signal (FE), and a center error signal (CE). The TE signal indicates the state of offset of a laser spot formed on disc surface 12-2 with respect to the center of a target track. Based on the TE signal, the servo control system exercises tracking control by controlling the converged laser beam to follow a track spiral on disk 12. The FE signal indicates the state of focus offset of OPU 18 with respect to disc surface 12-2. For example, the laser spot reflected back to photo-detector 20 generally has a circular form if lens 18 is in focus, and may have an elliptical form if lens 18 is out of focus. The CE signal indicates the state of offset of laser spot 22 formed on photo-detector 20 with respect to a central line 26 between photo-detector elements A, D and photo-detector elements B, C.
A technique in the art for generating a TE signal utilizes differential phase detection (“DPD”). FIG. 2 is a schematic view of a conventional circuit structure 30 according to DPD. Referring to FIG. 2, DPD structure 30 includes a photo-detector 32 having photo-detector elements A, B, C and D, amplifier pairs 34, equalizers 36, comparator pairs 38, a phase comparator 40, low pass filters 42 and a differential amplifier 44. However, mismatched amplifier gains between amplifier pairs 34 or mismatched hysteresis levels between comparator pairs 38 may cause different time lags. The difference between time lags may result in a TE signal distortion or cause a TE signal to swing, where the servo control performance deteriorates because of non-uniform zero-cross points when the swing TE signal is fixed-level sliced and sampled. A swinging TE signal includes a direct current (dc) component having a frequency, for example, smaller than one tenth of that of its alternating current (ac) component. The TE signal, designed to be fed back to the servo control system for better control in positioning the optical pickup, inherently includes an offset, i.e., the dc component.
It is thus desirable to provide circuits and methods to overcome at least the aforementioned shortcomings in the art and, more particularly, eliminate the offset to improve the TE signal. It is also desirable that the circuits and methods are applicable to other techniques in the art for generating TE signals, and to discs where tracks are recorded, unrecorded, or under recording, regardless of the state of the system being in a regulation mode or a search mode.